Time-lens System Research Articles

Thin-film lithium niobate circulator-free dispersion compensator in a time-lens system for femtosecond-pulse generation.

We demonstrate a circulator-free thin-film lithium niobate (TFLN) dispersion compensator based on the cascading 2 × 2 multimode interferometer (MMI) and two identical chirped Bragg gratings (CBGs). The cascaded MMI-CBG structure provides a dispersion value of 920 ps/nm/m over a 20 nm bandwidth covering 1537 to 1557 nm, featuring a compact footprint of 1 mm × 0.7 mm. Utilizing this device within a TFLN electro-optic time-lens system, we successfully generate 863-fs pulses at a 37 GHz repetition rate. Our compact, scalable, low-loss, and circulator-free dispersion compensator is the building block for the efficient generation of high-peak-power femtosecond laser pulses.

Fast-reconfigurable frequency comb generation based on AlGaAsOI waveguide with electro-optic time lens

Tunable optical frequency combs offer a flexible solution for specific applications such as dual-comb spectroscopy, optical communications and microwave photonics, delivering improved precision, compatibility, and performance. However, previously, there has been a trade-off between reconfigurability and system simplicity in comb generation. Here, we present a fast-switched repetition rate frequency comb system that utilizes an electro-optic modulation time-lens technique with a high third-order nonlinear AlGaAsOI waveguide. Only one stage of modulator is used in the time-lens system which significantly reduces the complexity of the overall system. Our system allows for tuning of the center wavelength from 1542 nm to 1556 nm, as well as independent adjustment of the repetition rates from 18 GHz to 26.5 GHz, enabling fast-switching capabilities. Additionally, our system exhibits a high pump-to-comb conversion efficiency of up to 67.9%. It also demonstrates robustness to temperature changes and environmental instability. All the involved devices can be integrated onto a single chip, making this comb suitable for various applications.

Synchronized time lens based temporal magnifier assisted by the phase lock loop.

We propose a synchronized time lens based temporal magnifier for the characterization of an ultrafast pulse, which is assisted by the phase lock loop (PLL) to actively lock the repetition rates of the pump laser and signal laser. A feedback control system combining a proportional-integral (PI) circuit and a piezoelectric transducer (PZT) inside the signal laser cavity is used to synchronize the repetition rates between the pump and signal lights. Benefitting from the PLL technique, the temporal position of the signal pulse remains as the numerical aperture of the time lens system, and therefore it has a high short-time stability for pulse measurement. This synchronized time lens based temporal magnifier can record single-shot pulses within continuous round trips. By engineering the dispersion of the pump, signal, and idler lights, we demonstrate a 200× magnification of the signal pulse. Our technique offers a simple synchronized method in a time lens system for ultrafast temporal characterization, which provides new insights into the observation of the fiber laser dynamics.

Spectral-temporal transient dynamics of dissipative soliton molecules in mode-locking fiber lasers

Dissipative soliton molecules that are analogous in many ways to matter molecules manifest complex evolution dynamics. Here, spectral-temporal transient internal motion dynamics of dissipative soliton molecules generated in a passively mode-locking erbium-doped fiber laser are experimentally investigated. Various categories of internal motion dynamics including attraction, creeping, dual-period breathing, and collision behaviors are revealed by employing an asynchronous time lens-based temporal magnifier together with the time stretched technique. Soliton temporal separation in those motion dynamics exhibit gradual decreasing and slightly wiggling motion. The temporal magnification factor of time lens system is substantially improved into 361. Our work makes the temporal magnifier more attractive for the applications of ultrafast measurements and nonlinear fiber optics, and benefits for fiber laser design.

Single-shot observation of breathers from noise-induced modulation instability using heterodyne temporal imaging.

We report phase and amplitude measurements of large coherent structures originating from the noise-induced modulation instability in optical fibers. By using a specifically designed time-lens system (SEAHORSE) in which aberrations are compensated, the complex field is recorded in single-shot over long durations of 200 ps with sub-picosecond resolution. Signatures of Akhmediev breather-like patterns are identified in the ultrafast temporal dynamics in very good agreement with numerical predictions based on the nonlinear Schrödinger equation.

Influences of group velocity dispersion on ultrafast pulse shaping in time lens

Time-lens technology is of significant interest in signal processing and optical communication. The impacts of group velocity dispersion (GVD) on ultrafast pulse shaping in a time-lens system based on four-wave mixing are explored in this paper. The output signals of temporal magnification and time-to-frequency conversion under different GVDs are theoretically investigated in detail. The simulation results imply that the femtosecond pulse is sensitive to GVD in propagation. GVD has an important effect on nonlinear parametric processes, which results in output signals presenting different pulse shapes and different frequency profiles. Furthermore, a model of silicon nitride waveguide with flat dispersion is proposed by finite element method and signal processing with negligible pulse distortion is realized in near-infrared region.

Generating ultrashort flat-top optical pulses with a fiber-loop time-lens system

For generating short flat-top optical pulses, traditional methods are either bandwidth limited or have low controllability (in pulse width and shape). We propose a fiber-loop based time-lens system to realize ps-level flat-top pulses, and theoretically study their features under various parameter circumstances. This time-lens system, functioning as many stacked time-lenses, introduces great controllability for the generated pulse’s characteristics through tuning both the phase modulation device and the dispersion-compensating setup. To achieve high-quality narrow flat-top pulses (with steep pulse edges), several factors including phase modulation function, modulation strength and dispersion compensating status must be optimized simultaneously. In simulation, after 50 loops of parabolic phase modulation and chirp compensation, the flat-top pulse’s duration can be narrowed to 2.8 ps with edge widths of 1.7 ps. This kind of ps-scale flat-top pulse is very desirable for application in various areas.

Generation and characteristics of shock optical pulses based on a fiber-loop time-lens system

<sec>The shock ignition scheme has the advantages of low ignition energy threshold, high gain, and good hydrodynamic stability, which has become one of the key schemes for the potentially successful ignition of inertial confinement fusion. The crucial element of shock ignition is how to achieve a highly efficient shock laser pulse. We propose a new scheme based on a time-lens system combining the fiber-loop phase modulation and the grating-pair compression to generate a highly controllable shock pulse. Based on the asymmetric phase modulation in time-domain followed by linear dispersion compensation in frequency domain, the shock pulse can be actively controlled with high precision in both pulse duration and pulse contrast (peak power ratio of the compression part to the shock part of the pulse). We construct a theoretical model based on the nonlinear Schrödinger equation to simulate the evolution of the spectrum and temporal shape of the shock laser pulse. The influences of various key parameters of the proposed system on the characteristics of the generated shock pulse are analyzed in depth. </sec><sec>The time lens system consists of three parts, i.e. the seed pulse carving part, the phase modulation loop, and the chirp-compensating grating pair. The operation principle of this system for generating shock pulse is as follows. First, a single-mode continuous wave 1053 nm distributed feedback seed laser is chopped into pulses with a Mach-Zehnder intensity modulator. Then the pulses enter into a fiber-loop for phase modulation. Owing to different modulation frequencies exerted on the left and right side of the pulse, the amount of spectral broadening of these two sides of the spectrum are also different after phase modulation. The spectrally broadened pulses are linearly chirped when the phase-modulation function has a parabolic shape. Finally, the pulse transits through a grating pair system for chirp compensating. Just like an anomalous dispersion delay line, the grating pair applies an anomalous group velocity dispersion to the passing optical pulse. When the chirp is compensated for appropriately, the pulse will be compressed. What the target pulse can be finally shaped into is dependent on the combined optimization of all the above processes.</sec><sec>The simulation results show that by systematically designing the parameters such as chopping function, phase modulation function, modulation depth, modulation frequency, and chirp compensating, the target shock pulse can be actively controlled with high-precision in the pulse width, pulse rising edge, and peak-power contrast. In addition, we can also tune only one parameter (such as the pulse width) of the pulse, with the other parameters kept unchanged. This new design idea and the proposed system can actively and independently adjust the two key parameters (the peak power contrast and the pulse width) of the generated shock pulse, which is not only helpful in deepening our understanding of the principle of laser-pulse shaping, but also significant for the subsequent practical implement of shock ignition of inertial confinement fusion.</sec>

An efficient way of third-order dispersion compensation for reshaping parabolic pulses through normal dispersion fiber amplifier

Normal dispersion decreasing fibers (NDDFs) employed for parabolic pulse (PP) generation are limited by the presence of inherent third-order dispersion (TOD), which leads to distortion in pulse shape from its parabolic nature. This necessitates a compensation method for TOD for such fibers. Dispersion-compensating fiber (DCF) amplifiers with relatively smaller values of dispersion can also be largely affected by this harmful effect of TOD when used for the same purpose. As a solution, a time-reversal system (TRS) based on a time-conjugating technique is employed with the aim of minimizing the deleterious effects of TOD. A detailed study on the estimation of the critical length where the time lens system needs to be implemented within the fiber link is crucial for reshaping the pulse into parabolic form. An NDDF is designed and optimized to possess a relatively higher value of TOD and a cascaded fiber optic system comprising the first stage of the proposed NDDF, the TRS and the second stage of the NDDF is suggested here to obtain linearly chirped PP at the output end even in the presence of a high value of TOD. The proposed scheme has also proved to be efficient for better pulse reshaping through DCFs possessing a high amount of TOD.

Single-shot measurement of phase and amplitude by using a heterodyne time-lens system and ultrafast digital time-holography

Temporal imaging systems are outstanding tools for single-shot observation of optical signals that have irregular and ultrafast dynamics. They allow long time windows to be recorded with femtosecond resolution, and do not rely on complex algorithms. However, simultaneous recording of amplitude and phase remains an open challenge for these systems. Here, we present a new heterodyne time-lens arrangement that efficiently records both the amplitude and phase of complex and random signals over large temporal windows (tens of picoseconds). Phase and time are encoded onto the two spatial dimensions of a camera. We implement this phase-sensitive time-lens system in two configurations: a time microscope and a digital temporal-holography device that enables single-shot measurement with a temporal resolution of ~80 fs. We demonstrate direct application of our heterodyne time-lens to turbulent-like optical fields and optical rogue waves generated from nonlinear propagation of partially coherent waves inside optical fibres. The use of a phase-sensitive time-lens system allows single-shot recording of both the amplitude and phase of random and complex signals with a high temporal resolution of ~80 fs over a long time window of ~40 ps.

The Application based on Fourier Transform in Time Lens

Using the similarity between the transmission of narrow band light pulses in the dispersion media and the transmission of spatial beam in paraxial diffraction, build a time lens system similar to the space lens. Space lens can achieve compression and amplification of imaging function, and it has the nature of Fourier transform in light field. Then the time lens can also realize the compression and amplification of the light pulse and its Fourier transform. Using the function of Fourier transform in the time domain of time lens system, achieved the effect of eliminating noise and removing side lobe, improved the white noise which generally exist in the communication system and the side lobe effect which will produce in the relevant processes.

Temporal Imaging of Incoherent-Light Intensity Waveforms Based on a Gated Time-Lens System

We experimentally demonstrate temporal imaging of incoherent-light intensity waveforms using a combination of a temporal gating process and a time lens. This design allows one to set the system temporal field of view and resolution in an independent fashion, thus overcoming the intrinsic time–bandwidth product tradeoff of previous incoherent-light schemes based on the temporal pinhole camera concept. In a proof-of-concept experiment, we report incoherent-light temporal compression of intensity waveforms by a factor of 2.86, with a time–bandwidth product exceeding 300, i.e., resolution of ∼26 ps (approximately fourfold improvement over the corresponding pinhole-only design) along a temporal field of view of ∼8 ns, using a basic linear-optics setup.

Time-lenses for time-division multiplexing of optical OFDM channels.

Time-lenses provide a promising platform for novel, broadband optical signal processing. However, in order to minimize system penalties, design constraints must be adequately taken into account. We investigate the impact of third-order-dispersion and nonlinear distortion on the performance of time-lens-based communication systems for the first time. Here, we propose a novel application of time-lenses - temporal compression and time-division multiplexing of optical OFDM channels, to provide a 1 Tb/s superchannel. Time-lens system performance degradations are investigated in our proposed system and the results are applicable to all four wave mixing based time-lens systems. Our work can help to optimize time-lens based communication systems.

Temporal Imaging of Optical Asymmetric Waveform Pulses With a Time Lens

In this paper, we investigated the imaging condition of asymmetric pulses on a temporal imaging (TI) system comprising two optical fibers and a phase modulator. We introduced a fitness function to evaluate the difference between target and output waveforms to quantitatively estimate TI characteristics. We successfully determined the optimal combination of phase modulation index and phase deviation to provide good TI by simulating the fitness function. We generated chirp-free pulses with symmetric and asymmetric waveforms using an in-line waveguide pulse synthesizer, i.e., an optical pulse synthesizer, based on pulse shaping and experimentally confirmed the TI condition in a time lens system. Moreover, TI was achieved not only at a theoretical value of the modulation index satisfying the TI condition but at a much smaller value than the theoretical value for a time-reversal mode as well. Measured waveforms of the experimental output agreed with simulation results. This may be important for practical applications because a TI system requires a much smaller modulation power.

Timing jitter in synchronized time-lens source for coherent Raman scattering imaging.

Synchronized time-lens source is suitable for various applications in coherent Raman scattering (CRS) imaging up to video-rate. Timing jitter between the time-lens source and the mode-locked laser is a crucial parameter for estimating the synchronization performance. Although it has been measured experimentally, there is a lack of theoretical investigation of this parameter. Here we demonstrate numerical simulation results of timing jitter in a synchronized time-lens system, with parameters similar to those in real experiments. Our results show that due to the optical delay between the time-lens source and the mode-locked laser, the timing jitter is close to the intrinsic timing jitter of the mode-locked laser. Further reduction in timing jitter can be achieved by matching the optical delays between the time-lens source and the mode-locked laser.

Ultrafast and versatile spectroscopy by temporal Fourier transform

One of the most remarkable and useful properties of a spatially converging lens system is its inherent ability to perform the Fourier transform; the same applies for the time-lens system. At the back focal plane of the time-lens, the spectral information can be instantaneously obtained in the time axis. By implementing temporal Fourier transform for spectroscopy applications, this time-lens-based architecture can provide orders of magnitude improvement over the state-of-art spatial-dispersion-based spectroscopy in terms of the frame rate. On the other hand, in addition to the single-lens structure, the multi-lens structures (e.g. telescope or wide-angle scope) will provide very versatile operating conditions. Leveraging the merit of instantaneous response, as well as the flexible lens structure, here we present a 100-MHz frame rate spectroscopy system – the parametric spectro-temporal analyzer (PASTA), which achieves 17 times zoom in/out ratio for different observation ranges.